1. Field of the Invention
This invention relates to an earplug, and in particular to a non-vented, multiple-resonator, noise attenuating earplug for insertion into the ear canal.
2. Description Relative to the Prior Art
The deleterious effects on the hearing capability of a person subjected to a high amplitude ambient noise field are well known. Personnel such as miners, construction workers, aircraft maintainers, military tank operators, pilots, as well as many industrial employees are routinely and necessarily subjected to loud noise environments while performing their duties. Preventing ambient sound from entering the ear canal protects users from excessive ambient noise that causes hearing damage. In addition, attenuating ambient noise is beneficial in communications systems because higher speech intelligibility results.
When an earplug is inserted into an ear canal, ambient sound is transmitted into the canal through movement of the earplug, through air leaks around the earplug, by sound impinging on the body and by traveling through skin tissue and bone into the canal, as well as by sound transmission through the earplug itself. Solid earplugs constructed of polymeric foam, wax, rubber, acrylic are currently available, and have no internal resonators. The attenuation of a well-sealed solid earplug generally increases monotonically as a function of rising frequency.
U.S. Pat. No. 3,047,089, issued to J. J. Zwislocki, discloses a non-vented passive earplug having a single internal resonator. A sound chamber along with its sound opening into the ear canal form an acoustic resonating system, known as a Helmholtz resonator. The resonant frequency, F0, of such a resonator is determined by the following formula:F0=1/(2π✓(M1C1))where M1 is the acoustic inertance at the sound opening and C1 is the compliance of the chamber volume. As an example, for a chamber volume of 2.5 cc with a compliance of 1.76×10−6 cm4 s2/g and the acoustic inertance is 0.01 g/cm4; the resonance frequency is 1200 Hz. Above the resonance frequency, the acoustic impedance at the sound opening is relatively high, and the earplug attenuation matches that of a solid earplug. Below resonance, the impedance is relatively small, and the chamber of the earplug increases the occluded ear canal volume. This results in enhanced attenuation at frequencies below resonance. The improvement in the attenuation, in decibels (dB), is calculated fromI=20 log((C1+Cc)/Cc)where Cc is the acoustic compliance of the occluded ear canal, which is approximately 8.60×10−7 cm4 s2 when using an earplug. Therefore, in this example the improved attenuation below resonance is approximately 10 dB. The resonance of the system is primarily damped by viscous losses associated with movement of air through the sound opening. A problem with the single-chamber earplug is its limited high-frequency attenuation performance. Also when a loudspeaker, or other sound transmitting means is used with the earplug to provide communications for the user, the sound pressure level developed using the loudspeaker is significantly reduced compared to a solid earplug also having an attached loudspeaker.
U.S. Pat. No. 2,347,490, issued to Watson et al, discloses an earplug made of flexible resilient material having two chambers separated by a partition containing a cotton-packed cylindrical insert having a minute hole in its endplate to allow static pressure equalization between the chambers. Watson et al do not disclose chambers having the structure of multiple acoustic resonators because the chamber coupled to the ear canal does not employ an inertial element, and accordingly, the disclosed earplug does not exhibit the attenuation characteristic of multiple acoustic resonance.
U.S. Pat. No. 5,832,094, issued to Le Her discloses an earplug with an acoustic valve. The earplug disclosed by Le Her is vented, in that there is an acoustic path provided through the earplug to the ear canal under normal operating conditions. The invention of Le Her selectively transmits ambient sound through the earplug and into the ear canal at all times. This sound transmission increases the unwanted sound level in the ear canal and diminishes the hearing protection function of the earplug.